Important to many types of circuits is the correct calibration of active filters. There are a number of techniques for calibrating such filters. For example, filter calibration techniques that use a proxy oscillator are well known to those skilled in the art of integrated circuit active filter design.
However, proxy oscillator methods suffer from a number of weaknesses. For example, in some cases, the proxy oscillator consumes power and possibly generates noise when the filter is enabled. Furthermore, there is an inherent mismatch between the proxy oscillator and the filter resulting in inaccuracies and inefficiencies being injected into the calibration process.
U.S. Pat. No. 5,245,646 of Jackson et al. describes a filter calibration circuit that uses an R/C integrator with known initial conditions, a timing circuit, and a decoding circuit to stabilize the global R/C time constant on an integrated circuit (IC). U.S. Pat. No. 5,914,633 of Comino et al. describes a similar calibration circuit. In both, the calibration circuit is disabled after calibration to address the problems of power consumption and noise generation. However, the problems created by the mismatch between the proxy oscillator and the filter remain.
U.S. Pat. No. 7,345,490 of Ibrahim et al. describes an RSSI-based filter calibration system wherein a signal is applied to the filter in the pass band and then at the 3 dB point. The filter control is then adjusted until the converted RSSI value of the signal in the pass band is 3 dB higher than the signal generated at the desired 3 dB point. However, this system requires additional circuitry with fine frequency resolution to generate both the signal in the pass band and the signal at the 3 dB bandwidth.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.